Peter Mandelson, the European Union's trade commissioner, yesterday waded into the row over BP's joint venture in Russia, describing the behaviour of BP's Russian partners as "rather menacing".

He said the dispute between BP and its Russian shareholders over the oil company TNK-BP was damaging Russia's reputation and raised questions about the country's investment climate.

"It's an essentially internal business issue. But there are two aspects that raise wider questions not just for the EU and Russia but for international investors," Mandelson told the Guardian yesterday. "There is the somewhat menacing behaviour of Russian shareholders and the use of state resources to apparently back up the Russian shareholders' position.

"That raises fears about the climate for international investment in Russia, and about the operation of the rule of law, and the relationship of the state and its agencies to what should be purely private-sector business rules."

BP, which owns half of the joint venture, is embroiled in an acrimonious battle with four Russian oligarchs who own the other half. The oligarchs, led by Mikhail Fridman, have demanded the resignation of TNK-BP chief executive Robert Dudley.

BP has come under pressure from the Russian authorities and state agencies - apparently acting at the instigation of the firm's Russian partners. The FSB - Russia's post-KGB spy agency - has twice raided BP's Moscow office and TNK-BP.

Officials have also summoned Dudley for questioning over taxes. Foreign staff seconded from BP have been locked out of the TNK-BP office for three months as part of the row, and have also had problems with visas.

Mandelson was in Russia yesterday for the EU-Russia summit in the western Siberian oil town of Khanty-Mansiisk. He said he raised the issue of TNK-BP with Russia's new president, Dmitry Medvedev.

"Mr Medvedev told us that the situation should not be dramatised, the positions of the partners should be resolved, and the state should be neutral. I welcome this," Mandelson said. "He emphasised that the state's job is to provide a legal framework and enforcement but otherwise keep away. This shows sense."

EU officials believe that the Russian shareholders will eventually sell their stake to Gazprom or another state-run entity. Their battle against BP is a ruthless attempt to drive up the multibillion-dollar price of any deal, they say.

The Russian billionaires - who hold their stake via the Alfa-Access-Renova (AAR) consortium - have already sued BP in Stockholm and in Russia. They accused the company of holding an illegal board meeting without them.

On Thursday the oligarchs - Fridman, Viktor Vekselberg, Len Blavatnik and German Khan - threatened new legal action against BP. They said they were considering action after the annual general meeting of TNK-BP Holding, the firm's main operating subsidiary, agreed a new list of board members.

The list included five BP representatives and four from AAR. The Russian shareholders said they had asked to be excluded from the new board.